Non-aqueous liquid electrolyte and lithium secondary battery comprising the same

ABSTRACT

The present invention relates to a lithium secondary battery comprising a non-aqueous liquid electrolyte comprising lithium bis(fluorosulfonyl)imide (LiFSI) and a fluorinated ether compound as additives, a positive electrode comprising a lithium-nickel-manganese-cobalt-based oxide as a positive electrode active material, a negative electrode and a separator. According to a non-aqueous liquid electrolyte for a lithium secondary battery of the present invention, a rigid SEI layer may be formed at a negative electrode during the initial charging of the lithium secondary battery comprising the same, the output properties of the lithium secondary battery may be improved, and the output properties after storing at high temperature and capacity properties may be increased.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Application No. PCT/KR2015/010239, filed Sep. 25, 2015which claims priority from Korean Patent Application No.10-2014-0128880, filed Sep. 26, 2014 and Korean Patent Application No.10-2015-0135260, filed Sep. 24, 2015, the disclosures of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a lithium secondary battery comprisinga non-aqueous liquid electrolyte comprising lithiumbis(fluorosulfonyl)imide (LiFSI) and a fluorinated ether compound asadditives, a positive electrode containing alithium-nickel-manganese-cobalt-based oxide as a positive electrodeactive material, a negative electrode and a separator.

BACKGROUND ART

According to the increase of technical development and demand on mobiledevices, the demand on secondary batteries as an energy source has beenrapidly increased. Among the secondary batteries, lithium secondarybatteries having high energy density and voltage are commerciallyavailable and widely used.

As a positive electrode active material of a lithium secondary battery,a lithium metal oxide is used, and as a negative electrode activematerial, a lithium metal, a lithium alloy, crystalline or amorphouscarbon or a carbon composite is used. The active material is coated on acurrent collector to an appropriate thickness and length, or the activematerial itself is coated as a film shape and then is wrapped or stackedwith a separator that is an insulating material, to form an electrodegroup. After that, the electrode group is inserted in a can or a vesselsimilar thereto, and an electrolyte is injected therein to manufacture asecondary battery.

In the lithium secondary battery, lithium ions repeat intercalation anddeintercalation from a lithium metal oxide of a positive electrode to acarbon electrode to conduct charging and discharging. In this case,lithium is strongly reactive and reacts with the carbon electrode toproduce Li₂CO₃, LiO, LiOH, etc. to form a coated layer on the surface ofa negative electrode. This coated layer called a solid electrolyteinterface (SET). The SEI layer formed at the beginning of charging mayprevent the reaction of the lithium ions with the carbon negativeelectrode or other materials during charging and discharging. Inaddition, the SEI layer performs the role of an ion tunnel and passesonly the lithium ions. The ion tunnel may induce the solvation of thelithium ions, and organic solvents of an electrolyte having highmolecular weight may induce co-intercalation at the carbon negativeelectrode, thereby preventing the breaking of the structure of thecarbon negative electrode.

Therefore, to improve the cycle properties at a high temperature and theoutput at a low temperature of a lithium secondary battery, a rigid SEIlayer is necessary to be formed at the negative electrode of the lithiumsecondary battery. Once the SEI layer is formed during an initialcharging, the SEI layer prevents the reaction of the lithium ions withthe negative electrode or other materials during repeating charging anddischarging while using the battery later and plays the role of the iontunnel for passing only the lithium ions between an electrolyte and thenegative electrode.

The improvement of the output properties at a low temperature is notexpected for a common electrolyte not comprising an electrolyte additiveor an electrolyte comprising an electrolyte additive with inferiorproperties due to the formation of a non-uniform SEI layer. In addition,even when an electrolyte additive is included, in the case when theamount required thereof is not controlled, the surface of the positiveelectrode may be decomposed during performing a reaction at a hightemperature due to the electrolyte additive, or an oxidation reaction ofthe electrolyte may be carried out, thereby increasing the irreversiblecapacity and deteriorating the output properties of a secondary battery.

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides a non-aqueous liquidelectrolyte for a lithium secondary battery, that may improve storageproperties at a high temperature and lifespan properties and a lithiumsecondary battery comprising the same.

Technical Solution

According to an aspect of the present invention, there is provided alithium secondary battery comprising a non-aqueous electrolytecomprising lithium bis(fluorosulfonyl)imide (LiFSI) and a fluorinatedether compound as additives, a positive electrode comprisinglithium-nickel-manganese-cobalt-based oxide as a positive electrodeactive material, a negative electrode and a separator.

The non-aqueous liquid electrolyte may further comprise a lithium salt,and a mixing ratio of the lithium salt and the lithiumbis(fluorosulfonyl)imide by molar ratio may be from 1:0.01 to 1:1. Theconcentration of the lithium bis(fluorosulfonyl)imide in the non-aqueousliquid electrolyte may be from 0.01 mol/L to 2 mol/L.

The lithium-nickel-manganese-cobalt-based oxide may be represented bythe following Formula 1.Li_(1+x)(Ni_(a)Co_(b)Mn_(c))O₂  [Formula 1]

In the above Formula, the conditions of 0.55≤a≤0.65, 0.18≤b≤0.22,0.18≤c≤0.22, −0.2≤x≤0.2 and x+a+b+c=1 may be satisfied.

Advantageous Effects

According to the non-aqueous liquid electrolyte for a lithium secondarybattery and the lithium secondary battery comprising the same, a rigidSET layer may be formed at a negative electrode during performing theinitial charging of the lithium secondary battery comprising the same,the thickness increase of a battery may be minimized by restraining thegeneration of a gas in high temperature environment, and preventing thedecomposition of the surface of a positive electrode and the oxidationreaction of an electrolyte, thereby improving the storing properties ata high temperature and the lifespan properties of the lithium secondarybattery.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail toassist the understanding of the present invention. It will be understoodthat terms or words used in the specification and claims, should not beinterpreted as having a meaning that is defined in dictionaries, butshould be interpreted as having a meaning that is consistent with theirmeaning in the context of the present invention on the basis of theprinciple that the concept of the terms may be appropriately defined bythe inventors for the best explanation of the invention.

The non-aqueous liquid electrolyte according to an embodiment of thepresent invention comprises lithium bis(fluorosulfonyl)imide (LiFSI).

The lithium bis(fluorosulfonyl)imide is added in the non-aqueous liquidelectrolyte as a lithium salt to form a rigid and thin SEI layer on anegative electrode and to improve output properties at a lowtemperature. Further, the decomposition of the surface of a positiveelectrode, which may be possibly generated during performing cycleoperation at a high temperature, may be restrained, and the oxidationreaction of the electrolyte may be prevented. In addition, since the SEIcoated layer formed on the negative electrode has a thin thickness, themovement of lithium ions at the negative electrode may be performedsmoothly, and the output of a secondary battery may be improved.

According to an embodiment of the present invention, the concentrationof the lithium bis(fluorosulfonyl)imide in the non-aqueous liquidelectrolyte is preferably from 0.01 mol/L to 2 mol/L and morepreferably, from 0.01 mol/L to 1 mol/L. In the case that theconcentration of the lithium bis(fluorosulfonyl)imide is less than 0.1mol/L, the improving effects of the output at a low temperature and thecycle properties at a high temperature may be insignificant, and in thecase that the concentration of the lithium bis(fluorosulfonyl)imideexceeds 2 mol/L, side reactions in the electrolyte during the chargingand discharging of the battery may occur excessively, swellingphenomenon may be generated, and the corrosion of a positive electrodeor a negative electrode collector formed by using a metal in theelectrolyte may be induced.

To prevent the above-described side reaction, a lithium salt may befurther included in the non-aqueous liquid electrolyte of the presentinvention. The lithium salt may comprise commonly used lithium salts inthis field. For example, one or a mixture of at least two selected fromthe group consisting of LiPF₆, LiAsF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiBF₄,LiSbF₆, LiN(C₂F₅SO₂)₂, LiAlO₄, LiAlCl₄, LiSO₃CF₃ and LiClO₄ may be used.

The mixing ratio of the lithium salt and the lithiumbis(fluorosulfonyl)imide is preferably from 1:0.01 to 1 by the molarratio. In the case that the mixing ratio of the lithium salt and thelithium bis(fluorosulfonyl)imide is greater than the upper limit, theside reaction in the electrolyte during the charging and discharging ofthe battery may be excessively carried out, and swelling phenomenon maybe generated. In the case that the molar ratio is less than the lowerlimit, the improvement of the output properties produced of thesecondary battery may be deteriorated. Particularly, in the case thatthe mixing ratio of the lithium salt and the lithiumbis(fluorosulfonyl)imide by the molar ratio is less than 1:0.01,irreversible reaction may be carried out a lot during the formingprocess of an SEI coated layer in a lithium ion battery and theintercalation process of solvated lithium ions by a carbonate-basedsolvent between negative electrodes, and the improving effects of theoutput at a low temperature and the cycle properties and capacityproperties after storing at a high temperature of the secondary batterymay become insignificant due to the exfoliation of the surface layer ofthe negative electrode (for example, the surface layer of carbon) andthe decomposition of an electrolyte. When the mixing ratio of thelithium salt and the lithium bis(fluorosulfonyl)imide by the molar ratioexceeds 1:1, excessive amount of lithium bis(fluorosulfonyl)imide may beincluded in an electrolyte, and an electrode collector may be corrodedduring performing charging and discharging and the stability of thesecondary battery may be deteriorated.

The positive electrode active material of thelithium-nickel-manganese-cobalt-based oxide may comprise an oxiderepresented by the following Formula 1.Li_(1+x)(Ni_(a)Co_(b)Mn_(c))O₂   [Formula 1]

In the above Formula, the conditions of 0.55≤a≤0.65 0.18≤b≤0.22,0.18≤c≤0.22, −0.2≤x≤0.2 and x+a+b+c=1 are satisfied.

By using the positive electrode active material of thelithium-nickel-manganese-cobalt-based oxide in the positive electrode,synergistic effect may be attained through the combination with thelithium bis(fluorosulfonyl)imide. When the amount of Ni in the positiveelectrode active material of the lithium-nickel-manganese-cobalt-basedoxide increases, cation mixing by which the site of Li⁺¹ ions and thesite of Ni⁺² ions are exchanged in the lamella structure of the positiveelectrode active material during charging and discharging may begenerated, and the structure thereof may be broken. Thus, the sidereaction of the positive electrode active material with the electrolytemay be performed, or the elution phenomenon of a transition metal may beexhibited. The cation mixing is carried out because the size of the Li⁺¹ion and the size of the Ni⁺² ion are similar. Through the side reaction,the electrolyte in the secondary battery may be depleted, and thestructure of the positive electrode active material may be broken,thereby easily deteriorating the performance of the battery.

Therefore, an electrolyte in which the lithium bis(fluorosulfonyl)imideis applied is used in the positive electrode active material of Formula1 according to an embodiment of the present invention to form a layer atthe surface of the positive electrode using a lithiumbis(fluorosulfonyl)imide induced component so as to restrain the cationmixing phenomenon of the Li ions and the Ni₊₂ ions while obtaining therange for securing the amount of a nickel transition metal sufficientfor securing the capacity of the positive electrode active material.According to the positive electrode active material comprising the oxideaccording to the above Formula 1 of the present invention, side reactionwith the electrolyte and metal eluting phenomenon may be effectivelyrestrained by using the electrolyte in which the lithiumbis(fluorosulfonyl)imide is applied.

In particular, in the case that the ratio of the Ni transition metal inthe oxide represented by the above Formula 1 exceeds 0.65, an excessiveamount of Ni is included in the positive electrode active material, andthe cation mixing phenomenon of the L⁺¹ ions and the Ni⁺² ions may notbe restrained even by the layer formed using the lithiumbis(fluorosulfonyl)imide at the surface of the electrode.

In addition, in the case that an excessive amount of the Ni transitionmetal is included in the positive electrode active material, theoxidation number of Ni may be changed. When the nickel transition metalhaving a d orbital makes a coordination bond, a regular octahedronstructure may be formed, however in an environment comprising a hightemperature, etc., the order of the energy level of the nickeltransition metal may be changed or the oxidation number thereof may bechanged (disproportionation reaction) by the application of externalenergy to form a twisted octahedron structure. Thus, the crystalstructure of the positive electrode active material comprising thenickel transition metal may be deformed, and the probability of theelution of the nickel metal in the positive electrode active materialmay be increased.

As a result, the inventors of the present invention confirmed that thehigh output, the stability at a high temperature and the good efficiencyof capacity properties may be secured through the combination of thepositive electrode active material comprising the oxide according to theabove Formula 1 with a lithium bis(fluorosulfonyl)imide salt.

In addition, a fluorinated ether compound may be included as anelectrolyte additive according to an embodiment of the presentinvention. Particularly, at least one selected from the group consistingof the compounds represented by the following Formula 2 may beillustrated.

In the above Formula 2, R₁ and R₂ are independently a linear or branchedalkyl group having 2 to 6 carbon atoms and at least 5 fluorine atoms.Particularly, the fluorinated ether compound may be at least oneselected from the group consisting ofdi(1,1,1,2,2,3,3,4,4-nonafluoropentyl)ether anddi(1,1,1,2,2,3,3,4,4,5,5-undecafluoropentyl)ether.

In a lithium secondary battery, oxygen released from a positiveelectrode in high temperature environment may promote the exothermicdecomposition reaction of an electrolyte solvent and induce theexpansion of a battery, so called, swelling phenomenon, to rapidlydeteriorate the lifespan and the efficiency of charging and dischargingof the battery. In some cases, the battery may be exploded and thestability thereof may be largely deteriorated. Since a fluorinesubstituent in the fluorinated ether compound added in the electrolyteis a flame retardant component, the generation of a gas due to thedecomposition of the electrolyte at a high temperature through thereaction of the electrolyte with the surface of the negative electrodeand the positive electrode in the battery may be restrained. Inaddition, the resistance in a voltage range of the secondary battery maybe decreased by using an ether compound that has low viscosity and mayincrease ionic conductivity. Thus, the lifespan properties of thesecondary battery may be improved.

In this case, the amount of the fluorinated ether compound may not belimited only if sufficient to accomplish the effects of the presentinvention comprising the improvement of the storing properties at a hightemperature and the lifespan properties of the battery. For example, theamount of the fluorinated ether compound may be from 1 to 20 wt % andpreferably may be from 3.0 to 15 wt % based on the total amount of theelectrolyte. In the case that the amount of the fluorinated ethercompound is less than 1 wt %, the restraining effect of gas generation,flame retardant properties and the decreasing effect of resistance maybe insufficiently obtained. In the case that the amount of thefluorinated ether compound exceeds 20 wt %, the increasing degree of theeffects may be limited, however irreversible capacity may be increasedor the resistance of the negative electrode may be increased.Particularly, the amount of the fluorinated ether compound may becontrolled by the amount added of the lithium bis(fluorosulfonyl)imideso as to efficiently prevent the generation of side reaction possiblycarried out according to the addition of a large amount of the lithiumbis(fluorosulfonyl)imide.

In addition, a non-aqueous organic solvent that may be included in thenon-aqueous liquid electrolyte is not limited only if the decompositionthereof due to oxidation reaction, etc. during the charging anddischarging of a battery may be minimized and target properties may beexhibited with the additive. For example, a nitrile-based solvent, acyclic carbonate solvent, a linear carbonate solvent, an ester solvent,an ether solvent or a ketone solvent, etc. may be used. These solventsmay be used alone or as a combination of two or more.

In the organic solvents, a carbonate-based organic solvent may bereadily used. The cyclic carbonate solvent may be one selected from thegroup consisting of ethylene carbonate (EC), propylene carbonate (PC)and butylene carbonate (BC), or a mixture of at least two thereof. Thelinear carbonate solvent may be one selected from the group consistingof dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate(DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) andethyl propyl carbonate (EPC), or a mixture of at least two thereof.

The nitrile-based solvent may be at least one selected from the groupconsisting of acetonitrile, propionitrile, butyronitrile, valeronitrile,caprylonitrile, heptanenitrile, cyclopentane carbonitrile, cyclohexanecarbonitrile, fluorobenzonitrile, 4-fluorobenzonitrile,difluorobenzonitrile, trifluorobenzonitrile, phenylacetoriitrile,2-fluorophenyl acetonitrile and 4-fluorophenyl acetonitrile. Thenon-aqueous solvent according to an embodiment of the present inventionmay be the acetonitrile.

Meanwhile, the lithium secondary battery according to an embodiment ofthe present invention may comprise a positive electrode, a negativeelectrode, a separator disposed between the positive electrode and thenegative electrode and the non-aqueous liquid electrolyte. The positiveelectrode and the negative electrode may comprise the positive electrodeactive material and the negative electrode active material,respectively, according to an embodiment of the present invention.

Meanwhile, the negative electrode active material may comprise amorphouscarbon and crystalloid carbon and may use carbon such as non-graphitizedcarbon, graphitized carbon, etc; a metal complex oxide such asLi_(x)Fe₂O₃ (0≤x≤1), Li_(x)WO₂ (0≤x≤1), Sn_(x)Me_(1−x)Me′_(y)O_(z) (Me:Mn, Fe, Ph and Ge; Me′: Al, B, P, Si, elements in group 1, group 2 andgroup 3 on the periodic table, and halogen; 0<x≤1; 1≤y≤3; 1≤z≤8), etc.;a lithium metal; a lithium alloy; a silicon-based alloy; a tin-basedalloy; an oxide such as SnO, SnO₂, PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃,Sb₂O₄, Sb₂O₅, GeO, GeO₂, Bi₂O₃, Bi₂O₄, Bi₂O₅, etc.; a conductive polymersuch as polyacetylene; and a Li—Co—Ni-based material.

In addition, the separator may be a porous polymer film, for example, aporous polymer film manufactured by using a polyolefin-based polymersuch as an ethylene homopolymer, a propylene homopolymer, anethylene/butane copolymer, an ethylene/hexene copolymer and anethylene/methacrylate copolymer alone, or a stacked film of at least twothereof. Besides, a commonly used porous non-woven fabric, for example,a non-woven fabric formed by using a glass fiber having a high meltingpoint, a polyethyleneterephthalate fiber, etc. may be used, withoutlimitation.

The secondary battery may have various types such as a cylindrical type,a prismatic type, a pouch type, etc. according to executing purpose, andthe present invention is not limited to the configuration known in thisart. The lithium secondary battery according to an embodiment of thepresent invention may be the pouch type secondary battery.

Mode for Carrying out the Invention

Hereinafter, the present invention will be explained in more detailreferring to examples and experimental examples, however the presentinvention is not limited to the following examples and experimentalexamples.

EXAMPLES Example 1

[Preparation of Electrolyte]

A non-aqueous liquid electrolyte was prepared by adding a non-aqueousorganic solvent having a component ratio of ethylene carbonate(EC):ethyl methyl carbonate (EMC)=3:7 (by volume), 0.9 mol/L of LiPF₆and 0.1 mol/L of lithium bis(fluorosulfonyl)imide as lithium salts basedon the total amount of the non-aqueous liquid electrolyte, and 5 wt % ofdi(1,1,1,2,2,3,3,4,4-nonafluoropentyl)ether as an additive based on thetotal amount of the non-aqueous liquid electrolyte.

[Manufacture of Lithium Secondary Battery]

92 wt % of Li (Ni_(0.6)Co_(0.2)Mn_(0.2))O₂ as a positive electrodeactive material, 4 wt % of carbon black as a conductive material, and 4wt % of polyvinylidene fluoride (PVdF) as a binder were added in anN-methyl-2-pyrrolidone (NMP) solvent to produce a positive electrodemixture slurry. The positive electrode mixture slurry was coated on apositive electrode collector, an aluminum (Al) thin film having athickness of 20 μm, dried and roll pressed to form a positive electrode.

In addition, a carbon powder as a negative electrode active material,PVdF as a binder, carbon black as a conductive agent were used in anamount ratios of 96 wt %, 3 wt % and 1 wt %, respectively, and wereadded in an NMP solvent to prepare a negative electrode mixture slurry.The negative electrode mixture slurry was coated on a negative electrodecollector, a copper (Cu) thin film having a thickness of 10 μm, driedand roll pressed to form a negative electrode.

A polymer type battery was manufactured using the positive electrode andthe negative electrode thus manufactured together with a separator withthree layers of polypropylene/polyethylene/polypropylene (PP/PE/PP), andthe non-aqueous liquid electrolyte prepared above was injected thereinto complete a lithium secondary battery.

Example 2

A non-aqueous liquid electrolyte and a lithium secondary battery wereobtained by performing the same procedure described in Example I exceptfor using 0.7 mol/L of LiPF₆ and 0.3 mol/L of lithiumbis(fluorosulfonyl)imide as lithium salts based on the total amount ofthe non-aqueous liquid electrolyte.

Example 3

A non-aqueous liquid electrolyte and a lithium secondary battery wereobtained by performing the same procedure described in Example 1 exceptfor using 0.6 mol/L of LiPF₆ and 0.4 mol/L of lithiumbis(fluorosulfonyl)imide as lithium salts based on the total amount ofthe non-aqueous liquid electrolyte.

Example 4

A non-aqueous liquid electrolyte and a lithium secondary battery wereobtained by performing the same procedure described in Example I exceptfor using 0.5 mol/L of LiPF₆ and 0.5 mol/L of lithiumbis(fluorosulfonyl)imide as lithium salts based on the total amount ofthe non-aqueous liquid electrolyte.

Example 5

A non-aqueous liquid electrolyte and a lithium secondary battery wereobtained by performing the same procedure described in Example 1 exceptfor using di(1,1,1,2,2,3,3,4,4,5,5-undecafluoropentyl)ether, instead ofdi(1,1,1,2,2,3,3,4,4-nonafluoropentyl)ether.

Example 6

A non-aqueous liquid electrolyte and a lithium secondary battery wereobtained by performing the same procedure described in Example I exceptfor using CF₃CH₂OCF₂CF₂H (AE3000, Asahi Glass Co., Ltd.), instead ofdi(1,1,1,2,2,3,3,4,4-nonafluoropentyl)ether.

Comparative Example 1

A non-aqueous liquid electrolyte and lithium secondary battery wereobtained by performing the same procedure described in Example I exceptfor using 0.4 mol/L of LiPF₆ and 0.6 mol/L of lithiumbis(fluorosulfonyl)imide as lithium salts based on the total amount ofthe non-aqueous liquid electrolyte.

Comparative Example 2

A non-aqueous liquid electrolyte and a lithium secondary battery wereobtained by performing the same procedure described in Example 2 exceptfor not adding an additive.

Comparative Example 3

A non-aqueous liquid electrolyte and a lithium secondary battery wereobtained performing the same procedure described in Example 2 except forusing Li(Ni_(0.5)Co_(0.3)Mn_(0.2))O₂ as the positive electrode activematerial.

Experimental Examples

<Output Properties after Storing at High Temperature>

The secondary batteries manufactured in Examples 1 to 4 and ComparativeExamples 1 to 3 were stored at 60° C. for 16 weeks, and the outputthereof was calculated using voltage difference generated when chargingand discharging at 23° C. with 5 C for 10 seconds. The output after 16weeks was calculated by percent based on an initial output (output after16 weeks (W)/initial output (W)*100(%)), and the results are illustratedin the following Table 1. The test was performed at the state of charge(SOC) of 50%.

<Capacity Properties after Storing at High Temperature>

The secondary batteries manufactured in Examples 1 to 4 and ComparativeExamples 1 to 3 were charged in constant current/constant voltage(CC/CV) conditions to 4.2 V/38 mA with 1 C and discharged in CCconditions to 2.5 V with 3 C, and the discharge capacity thereof wasmeasured. After that, the secondary batteries manufactured in Examples 1to 4 and Comparative Examples 1 to 3 were stored at 60° C. for 16 weeks,charged in constant current/constant voltage (CC/CV) conditions at 23°C. to 4.2 V/38 mA with 1 C and discharged in CC conditions to 2.5 V with3 C. The discharge capacity after weeks was calculated by percent basedon an initial discharge capacity (discharge capacity after 16weeks/initial discharge capacity*100(%)), and the results areillustrated in the following Table 1.

<Measuring of Thickness of Battery>

The secondary batteries manufactured in Examples 1 to 4 and ComparativeExamples 1 to 3 were stored at 60° C. for 16 weeks, and thicknessincreasing rate (%) with respect to an initial thickness of the batterywas measured. The results are illustrated in the following Table 1.

TABLE 1 Properties after scoring at high temperature Battery thicknessOutput Capacity increasing properties (%) properties (%) rate (%)Example 1 95.4 89.9 5.3 Example 2 96.7 92.8 4.1 Example 3 96.8 91.4 4.9Example 4 96.0 90.9 5.2 Example 5 96.3 91.6 4.3 Example 6 93.9 90.8 6.8Comparative 95.2 88.7 6.1 Example 1 Comparative 92.4 84.1 19.7 Example 2Comparative 88.6 80.5 9.8 Example 3

As shown in Table 1, since the secondary batteries of Examples 1 to 4use the fluorinated ether compound as an additive, the stability thereofat high temperature may increase, and the increasing rate of resistancethereof may decrease. Therefore, the properties after storing at hightemperature (capacity and output properties) may be good via thecombination with the lithium salt of the lithiumbis(fluorosulfonyl)imide when compared to that of the secondarybatteries of Comparative Examples 1 to 3.

Meanwhile, since the fluorinated ether compound was not used inComparative Example 2, the thickness increasing rate after storing athigh temperature was 19.7% and it means the battery has excessivelyswollen. In addition, the capacity properties and the output propertiesof the secondary battery of Example 6 were inferior to those of thesecondary batteries of Example 1 to 5, although the fluorinated ethercompound was used as an additive in the secondary battery of Example 6.The reason for the inferior capacity property and output property of thesecondary battery of Example 6 considered as follows: The fluorinatedether compound included in the secondary battery of Example 6 has analkyl group having 5 or less fluorine independently on both sides ofoxygen atom, whereas the fluorinated ether compounds included in thesecondary batteries of Example 1 to 5 have an alkyl group having 5 ormore fluorine independently both sides of oxygen atom.

<Lifespan Properties at Room Temperature>

The secondary batteries manufactured in Examples 1 to and ComparativeExamples 1 to 3 were charged in CC/CV conditions at 23° C. to 4.2 V/38mA with C and discharged in CC conditions to 2.5 V with 3 C, and thedischarge capacity thereof was measured. This experiment was repeatedlyperformed from 1^(st) to 800^(th) cycles. The discharge capacity at the800^(th) cycle was calculated by percent based on the capacity at the 1°cycle (capacity at 800^(th) cycle/capacity at 1^(st) cycle*100(%)), anddata thus obtained are illustrated in the following Table 2.

<Lifespan Properties at High Temperature>

The secondary batteries manufactured in Examples 1 to and ComparativeExamples 1 to 3 were charged in CC/CV conditions at 45° C. to 4.2 V/38mA with 1 C and discharged in CC conditions to 2.5 V with 3 C, and thedischarge capacity thereof was measured. This experiment was repeatedlyperformed from 1^(st) to 800^(th) cycles. The discharge capacitymeasured at the 800^(th) cycle was calculated by percent based on thecapacity at the 1^(st) cycle (capacity at 800^(th) cycle/capacity at1^(st) cycle*100(%)), and data thus obtained are illustrated in thefollowing Table 2.

TABLE 2 Lifespan properties (%) Lifespan properties Lifespan propertiesat room temperature at high temperature Example 1 84.9 81.1 Example 288.7 84.8 Example 3 87.4 83.3 Example 4 85.9 81.7 Example 5 87.1 84.2Example 6 82.6 76.5 Comparative 82.1 78.8 Example 1 Comparative 77.469.9 Example 2 Comparative 69.1 61.7 Example 3

As shown in Table 2, it would be secured that the lifespan properties atroom temperature and at high temperature of the lithium secondarybatteries of Examples 1 to 4 are better than those of the lithiumsecondary batteries of Comparative Examples 1 to 3. The lifespanproperties at high temperature and the lfespan properties at roomtemperature of the lithium secondary battery of Comparative Example 3using Li(Ni_(0.5)Co_(0.3)Mn_(0.2))O₂ as the positive electrode activematerial are found markedly low.

Meanwhile, the lifespan properties at room temperature and at hightemperature of the lithium secondary batteries of Examples 6 areinferior to those of the lithium secondary batteries of Examples 1 to 5,due to that the fluorinated ether compound included in the secondarybattery of Example 6 has an alkyl group having 5 or less fluorineindependently on both sides of oxygen atom.

The invention claimed is:
 1. A lithium secondary battery comprising: anon-aqueous liquid electrolyte comprising lithiumbis(fluorosulfonyl)imide (LiFSI) and a fluorinated ether compound asadditives; a positive electrode comprising alithium-nickel-manganese-cobalt-based oxide as a positive electrodeactive material; a negative electrode; and a separator, wherein thelithium-nickel-manganese-cobalt-based oxide is represented by thefollowing Formula 1:Li_(1+x)(Ni_(a)Co_(b)Mn _(c))O₂   [Formula 1] in the above Formula,0.6≤a≤0.65, 0.18≤b≤0.22, 0.18≤c≤0.22, −0.2≤x≤0.2 and x+a+b+c=1.
 2. Thelithium secondary battery of claim 1, wherein the non-aqueous liquidelectrolyte further comprises a lithium salt.
 3. The lithium secondarybattery of claim 2, wherein a mixing ratio of the lithium salt and thelithium bis(fluorosulfonyl)imide by molar ratio is from 1:0.01 to 1:1.4. The lithium secondary battery of claim 1, wherein a concentration ofthe lithium bis(fluorosulfonyl)imide in the non-aqueous liquidelectrolyte is from 0.01 mol/L to 2 mol/L.
 5. The lithium secondarybattery of claim 2, wherein the lithium salt is one or a mixture of atleast two selected from the group consisting of LiPF₆, LiAsF₆, LiCF₃SO₃,LiN(CF₃SO₂)₂, LiBF₄, LiSbF₆, LiN(C₂F₅SO₂)₂, LiAlO₄, LiAlCl₄ and LiClO₄.6. The lithium secondary battery of claim 1, wherein the fluorinatedether compound is at least one selected from compounds represented bythe following Formula 2:

where R₁ and R₂ are independently a linear or branched alkyl grouphaving 2 to 6 carbon atoms and at least 5 fluorine atoms.
 7. The lithiumsecondary battery of claim 1, wherein the fluorinated ether compound isat least one selected from the group consisting ofdi(1,1,1,2,2,3,3,4,4-nonafluoropentyl)ether anddi(1,1,1,2,2,3,3,4,4,5,5-undecafluoropentyl)ether.
 8. The lithiumsecondary battery of claim 1, wherein an amount of the fluorinated ethercompound is 1-20 wt % based on a total amount of the non-aqueous liquidelectrolyte.
 9. The lithium secondary battery of claim 1, wherein thenon-aqueous liquid electrolyte further comprises a non-aqueous organicsolvent selected from the group consisting of a nitrile-based solvent, alinear carbonate solvent, a cyclic carbonate solvent, an ester solvent,an ether solvent, a ketone solvent and a combination thereof.
 10. Thelithium secondary battery of claim 9, wherein the cyclic carbonatesolvent is one or a mixture of at least two selected from ethylenecarbonate (EC), propylene carbonate (PC) and butylene carbonate (BC),and the linear carbonate solvent is one or a mixture of at least twoselected from the group consisting of dimethyl carbonate (DMC), diethylcarbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC),methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC).
 11. Thelithium secondary battery of claim 9, wherein the nitrile-based solventis at least one selected from the group consisting of acetonitrile,propionitrile, butyronitrile, valeronitrile, caprylonitrile,heptanenitrile, cyclopentane carbonitrile, cyclohexane carbonitrile,2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile,trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenyl acetonitrileand 4-fluorophenyl acetonitrile.
 12. The lithium secondary batteryaccording to claim 1, wherein the secondary battery is a pouch typelithium secondary battery.